<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .header {
            width: 500px;
            height: 100px;
            border: 5px solid black;
            /* border-color: cadetblue;
        border-width: 5px;
        border-style: ; */
        }

        .first {
            font-size: 30px;
        }

        #content {
            width: 100%;
            height: 200px;
            background-color: cadetblue;
        }
        div p{
            color:red;
        }
    </style>
</head>

<body>
    <div class="header first">类选择器：格式为 &lt;标签 class="类名"&gt; </div>

    <div class="header" id="content">类选择器：格式为 &lt;标签 class="类名"&gt; </div>
    <div id="content">id选择器：格式为 &lt;标签 class="id名"&gt; </div>
    <div id="content">id选择器：格式为 &lt;标签 class="id名"&gt; </div>
    <div>
        <p>
            这个p标签有样式
        </p>
    </div>
    <p>这个标签没有样式</p>
</body>

</html>